[banzai]

Hello people


Is the main purpose for grounding the electrical system to make protection devices like CB's work ?, why not use ungrounded systems like the one we get from an isolation transformers secondary that is not grounded like the primary, wouldn't that be safer because you will have to touch both wires to get electrocuted instead of just touching one ?

Is it because it would be harder to detect a fault without a ground because we wouldn't be able to detect the fault instantly unless the hot touches the N (short circuit) ?

What is the main purposes of grounding an system, is one of them to detect a fault rapidly, ?


Enlighten me


PS

I'm freshly new to the field and here to learn from the experienced ones.

Happy New Year

 

[Dennis Alwon]

The cb will work without a ground but the grounding will help if there were current leakage to ground thus opening the CB. I am not familiar with the ungrounded systems although I understand how it works, but there must be a reason we don't use them.

 

[Big John]

The problem with an ungrounded system is that in many cases despite being ungrounded it will not provide the same protection you may have seen for example from a small isolation transformer.

Since no insulator is perfect, every wire has a little resistive current flow through the insulation. Also, because we use alternating current, every conductor has reactive current flow and is coupled to neighboring conductors (including the earth). When you start adding both these currents up for enough conductors, they can result in hundreds of milliamps or even dozens of amps worth of ground current in an ungrounded system.

Since it only takes 100-200mA of current to electrocute you, and far less than that to deliver a painful shock, having an ungrounded system doesn't protect you.

The other issue comes from fault detection and clearance: In a grounded system, the "detection and clearance" method is just a wire and a circuit breaker. It's simple and reliable. In an ungrounded system, GF detection technology is much more complicated and as a result less reliable. When you consider that ground faults are the most common form of short-circuit, you want the most reliability you can get out of your protection.

The final issue is that in an ungrounded system of a large enough size, an intermittent ground fault can be damaging. Without a ground reference, your electrical system is a giant capacitor: The earth is one plate, the wires are another, with the insulation in between. During a restriking ground fault, the collapsing electrical field in this capacitance will contribute to the system voltage and cause significant over-voltages. In a house with a small electrical system, I can't say how destructive these transients might be, but in a large industrial environment I've seen this type of fault blow holes in a whole lot of motor and generator insulation.

So, long-story-short, we ground because it stabilizes voltages and provides a reliable method of clearing ground-faults when they occur.

That said, there's an absolute ton of hocus-pocus surrounding it, and I would argue that we over-ground 99% of the buildings we build.

 

[KGN742003]

There are ungrounded systems, they are just more scarce now. We bond the metal parts in a building and the Neutral(or phase conductor) to the ground to provide a path back to the source to trip a breaker or a fuse in a fault condition. We run grounding electrodes and conductors to mitigate things like lightening strikes.

 

[newbi]

Are we talking about grounding or bonding here ?

 

[Meadow]

Are we talking about grounding or bonding here ?

Same thing Im thinking.

 

[Meadow]

The problem with an ungrounded system is that in many cases despite being ungrounded it will not provide the same protection you may have seen for example from a small isolation transformer.

Since no insulator is perfect, every wire has a little resistive current flow through the insulation. Also, because we use alternating current, every conductor has reactive current flow and is coupled to neighboring conductors (including the earth). When you start adding both these currents up for enough conductors, they can result in hundreds of milliamps or even dozens of amps worth of ground current in an ungrounded system.

Since it only takes 100-200mA of current to electrocute you, and far less than that to deliver a painful shock, having an ungrounded system doesn't protect you.

The other issue comes from fault detection and clearance: In a grounded system, the "detection and clearance" method is just a wire and a circuit breaker. It's simple and reliable. In an ungrounded system, GF detection technology is much more complicated and as a result less reliable. When you consider that ground faults are the most common form of short-circuit, you want the most reliability you can get out of your protection.

The final issue is that in an ungrounded system of a large enough size, an intermittent ground fault can be damaging. Without a ground reference, your electrical system is a giant capacitor: The earth is one plate, the wires are another, with the insulation in between. During a restriking ground fault, the collapsing electrical field in this capacitance will contribute to the system voltage and cause significant over-voltages. In a house with a small electrical system, I can't say how destructive these transients might be, but in a large industrial environment I've seen this type of fault blow holes in a whole lot of motor and generator insulation.

So, long-story-short, we ground because it stabilizes voltages and provides a reliable method of clearing ground-faults when they occur.

That said, there's an absolute ton of hocus-pocus surrounding it, and I would argue that we over-ground 99% of the buildings we build.

Depending on the design. Maybe not with many yards of wire or high voltages, but ungrounded systems are the norm in Hospital ORs and intensive care units both in the US and overseas. They are built to have a leakage current of no more than 3 or 6ma depending on the requirements.

 

[banzai]

but ungrounded systems are the norm in Hospital ORs and intensive care units both in the US and overseas

Is that done for the safety or because they use sensitive devices ?

 

[Meadow]

Is that done for the safety or because they use sensitive devices ?

Its done to maintain service continuity should on phase become grounded power will still flow. Also should someone come between a phase and a ground minimal current will flow. Bonding is exceptionally well everything is at equal potential however after the isolation transformer both the phases remain floating. A line isolation monitor makes sure no phases are faulted.

If really curious in Norway ungrounded systems are the norm. Power comes from a 3 phase 138/240 volt transformer. The XO is grounded via a impact protection or surge arrestor. It keeps the XO floating, however if it a surge occurs such as a primary crossing into the secondary an MOV in the unit shunts to ground and if the fault is permanent a fuse blows in the surge arrester that will cause a set of contacts to permanently short the XO to ground. Houses are straight 240 volts with 2 or 3 phases and newer houses all have a ground wire as well that bonds all the services together.


Ungrounded was the norm in factories in the US where service continuity is required however high resistance grounding is taking over since a well designed HRG system limits surges from arcing ground faults.

 

[Semi-Ret Electrician]

Since an operating room is constantly connecting stuff up to a patient's heart, brain etc. that "stuff" must have zero fault current.

Unlike the resistance our hands etc offer.

 

[macmikeman]

Big John answered that in a way that a whole lot of big guns in this industry were not able to when I confronted them with the same simple question this first year man just asked us. Thank you John. I used to ask them What does Stablilize to Ground mean exactly, and they always used to get it wrong.....






Make Johns post a sticky, some of Mike Holts experts might learn if they wander over here by accident.....

 

[don_resqcapt19]

...
So, long-story-short, we ground because it stabilizes voltages and provides a reliable method of clearing ground-faults when they occur.
...

In reality the "grounding" (connection to earth) has very little to do with clearing "ground faults". You need a fault clearing path back to the source to clear those faults. That path is via the EGCs, the main bonding jumper and the grounded conductor. The connection to earth will provide a parallel path for the ground fault current, but that connection will most often have an impedance several orders of magnitude higher than the code intended fault clearing path.

 

[Big John]

Depending on the design. Maybe not with many yards of wire or high voltages, but ungrounded systems are the norm in Hospital ORs and intensive care units both in the US and overseas. They are built to have a leakage current of no more than 3 or 6ma depending on the requirements.

I know they exist, but I've never worked on them, so I don't know how the design differs from large ungrounded systems other than size.

It's possible I suppose that a small ungrounded system in a house would operate similarly, but I've seen more than one GFCI trip from the cumulative leakages caused by many over-driven staples, so I'm betting there would still be a shock hazard.

In reality the "grounding" (connection to earth) has very little to do with clearing "ground faults". You need a fault clearing path back to the source to clear those faults....

No argument, but as you know in an ungrounded system the first ground fault is only half of a short-circuit. So if the goal is to reliably and quickly eliminate any ground faults, ungrounded systems are not ideal.

 

[brian john]

I know they exists, but I've never worked on them, so I don't know how the design differs from large ungrounded systems other than size.

In theory and general practice they are a very good system but in general use it would be a nightmare. No commercial management firm would spend the money required to keep the system operational. Plus leakage current increases as the size of the distribution system increases.

In hospitals the UG systems are monitored and the equipment used in the OR tested regularly.

As hospitals get bought up by big corporations and budgets squeezed I would bet you see less money spent on maintenance. We discussed this last night we use to do a slew of hospital testing and maintenance that long list has been shortened in the less decade.

http://static.schneider-electric.us/assets/pdf/healthcare/Isolated Power Systems 4800CT9801.pdf

As for UG systems in commercial installations while safer, less arcing round Faults on the first fault. We see grounded systems with multiple grounds due to accidental and/or intentional connections of the neutral downstream from the main neutral ground bond.

 

[Meadow]

I know they exist, but I've never worked on them, so I don't know how the design differs from large ungrounded systems other than size.

It's possible I suppose that a small ungrounded system in a house would operate similarly, but I've seen more than one GFCI trip from the cumulative leakages caused by many over-driven staples, so I'm betting there would still be a shock hazard. No argument, but as you know in an ungrounded system the first ground fault is only half of a short-circuit. So if the goal is to reliably and quickly eliminate any ground faults, ungrounded systems are not ideal.

A properly designed isolated ground is far from a shock hazard. Usually fed from a separate panel near the source with an isolation transformer built in. Only 120 volt loads are served. When the system is installed XHHW conductors are used in conduit since it has a thicker jacket of THHN. Runs are kept short and meggered. The end result is often less than 3ma of fault current. You can touch either conductor and not feel anything. A line isolation monitor looks for insulation break downs or faults. The second reason for their use would be not to quickly clear the fault but wait until it its safe to shut the circuit down.

These are nothing like resi construction that in my opinion needs a material and labor overhaul.

 

[banzai]

In an ungrounded system, GF detection technology is much more complicated and as a result less reliable.

How can you have ground faults when you are being supplied with power from an ungrounded system/transfomer ?

 

[magmash]

I have seen ungrounded systems used in places like grain elevators, all the lightning and electrical sockets is supplied via a 1:1 transfomer, by using such systems there is a less risk of getting an electric arc from an earth fault

 

[macmikeman]

In reality the "grounding" (connection to earth) has very little to do with clearing "ground faults". You need a fault clearing path back to the source to clear those faults. That path is via the EGCs, the main bonding jumper and the grounded conductor. The connection to earth will provide a parallel path for the ground fault current, but that connection will most often have an impedance several orders of magnitude higher than the code intended fault clearing path.

When ever I make that statement , I precipice it with "below 600 volts" A ten million volt ground fault probably clears pretty quick thru earth used as a path back to source.

 

[RIVETER]

Hello people


Is the main purpose for grounding the electrical system to make protection devices like CB's work ?, why not use ungrounded systems like the one we get from an isolation transformers secondary that is not grounded like the primary, wouldn't that be safer because you will have to touch both wires to get electrocuted instead of just touching one ?

Is it because it would be harder to detect a fault without a ground because we wouldn't be able to detect the fault instantly unless the hot touches the N (short circuit) ?

What is the main purposes of grounding an system, is one of them to detect a fault rapidly, ?


Enlighten me


PS

I'm freshly new to the field and here to learn from the experienced ones.

Happy New Year

The MAIN reasons that we ground an electrical system is to protect THAT electrical system in the event of a lightning strike, or, in the rare event of a primary to secondary short at the service transformer. It has not a thing to do with allowing the circuit breakers to trip.

 

[Meadow]

The MAIN reasons that we ground an electrical system is to protect THAT electrical system in the event of a lightning strike, or, in the rare event of a primary to secondary short at the service transformer. It has not a thing to do with allowing the circuit breakers to trip.

I agree. When an ungrounded system is used on site I forget the exact requirements the NEC requires a shield between the primary and secondary to prevent the primary crossing into the LV secondary. Even in other parts of the world where ungrounded power is used either the XO or a phase needs to be grounded through "impact protection", a device to short to ground during a HV or MV to LV fault.